Objective-C. Smalltalk C++ Objective-C. Java C#

Transcription

1 Objective-C

2 Objective-C Smalltalk C Objective-C C++ Java C# Strict superset of C o You can mix C with Objective-C o Can even mix C++ with Objective-C (usually referred to as Objective-C++) Language is relatively simple compared to C++, but does introduce some new syntax

3 Similarities to C/C++ Most primitive data types Variable declaration and initialization Assignment statements and arithmetic Conditional expressions and flow control statements (loops, decisions) Standalone functions (almost never used in practice) Arrays (almost never used in practice use NSArray class instead) C strings (almost never used in practice use NSString class instead) Supports both C-style and C++ style comments Key Differences from C++ Requires a runtime environment (like Java or C#) No using statements / namespaces main() returns void Sizes of built-in types are not standardized, but platform-independent versions are available No operator overloading No function or method overloading a class may not contain two methods with the same name No concept of const correctness Syntax for declaring classes is different Syntax for method declarations and calling methods is very different (derived from Smalltalk messaging) No constructors or destructors Single inheritance classes inherit from one and only one superclass No abstract classes

4 Syntax Additions Protocols define behaviors that cross classes (similar to interfaces in Java or C#) Categories allow you to add new methods to an existing class (as an alternative to subclassing) Some new data types o Generic object type o Class o Selectors Dynamic Runtime Object creation o All objects allocated from heap o No stack based objects Dynamic typing data type of an object can be determined at runtime (introspection) Dynamic binding which version of a method to call is determined at runtime Dynamic loading parts of a program can be loaded into memory at runtime as needed Requires a runtime environment (like Java or C#)

5 Objects An object associates data (state) with the particular operations that can use or affect that data (behavior) Behavior is implemented using methods State is maintained using instance variables Instance variables are internal to the object o State of the object is usually accessible only through the object s methods o Getter/setter methods are often created specifically to access instance variables Object sees only the methods that were designed for it Classes and Instances Objects are defined by defining their class A class definition is a prototype for a kind of object; it declares instance variables and defines methods that all objects of the class can use A class itself is an object in Objective-C o For each class in a program, the compiler creates one class object that knows how to build new objects belonging to that class o A class object is the compiled version of the class; the objects it builds are instances of the class Instances respond to instance methods o Instance methods start with a - o These are the normal methods you are used to o They can access instance variables o They can send messages to self and super inside The keyword self refers to the object that is the receiver of the current method The keyword super refers to the superclass of the message receiver

6 Class objects respond to class methods o Class methods start with a + o Used for memory allocation, singletons, utilities o Can not access instance variables o Messages to self and super mean something a little different o Can only access other class methods OOP Principles Encapsulation o Bundle data with the methods that operate on that data o Keep implementation hidden and separate from interface Polymorphism o Different objects, same interface Inheritance o Base new classes on previously created classes; reuse code, extand or customize behaviors o Inheritance relationships give rise to a hierarchical organization

7 Inheritance Superclass NSObject UIResponder UIView Memory management Generic behaviors UIControl Subclass UIButton UITextField Specific behaviors There is a hierarchical is a relationship exists between subclass and its superclasses o A UIButton is a UIControl (and a UIView, and a UIResponder, and an NSObject) Subclasses inherit behavior and data from their superclasses Subclasses can add new data and methods Subclasses can use, augment, or replace superclass methods

8 Defining a Class In Objective-C, classes are defined in two parts: 1. A public interface that declares the methods and instance variables of the class and names its superclass. Header File 2. A private implementation that actually defines the class (contains the code that implements its methods). Implementation File Sometimes a class definition spans several files through the use of a feature called a category.

9 Class Interface Usually placed in a header file. The declaration of a class interface begins with the compiler and ends with the Class name Superclass name (optional) Protocols (optional) Instance variable declarations Method MyClass : NSObject <ProtocolA, ProtocolB> int count; id data; // Object reference - weak typing NSString *name; // Object reference - strong typing - (id)initwithstring:(nsstring *)aname; + (MyClass *)createmyclasswithstring:(nsstring If colon and superclass name are omitted, new class is declared as a root class. In addition to methods, classes can also declare properties.

10 Access Modifiers and Variables Objective-C supports the following acess modifiers for instance variables: Directive The instance variable is accessible only within the class that declares (Default) The instance variable is accessible within the class that declares it and within classes that inherit The instance variable is accessible We won t worry about this level, since it s mostly useful for framework classes. MyObject : NSObject int int int int height; int int y; foo and bar are protected x and y are private height and width are public

11 Method Declarations Same information as in a C++ method prototype, but syntax is different: Return data type Method takes argument Argument name - (void)setgpa:(double)newgpa; Method type Method name Argument data type Method type is either + (for a class method) or (for an instance method) Some examples: - (void)enrollat:(nsstring *)university withmajor:(nsstring *)major; - (double)gpa; - (void)setgpa:(double)newgpa; - (BOOL)canGraduate; - (Person *)advisor; - (NSString *)name;

12 Importing an Interface The interface file for a class must be included in any source module (header or implementation file) that depends on the class interface o Any module that creates an instance of the class o Any module that sends a message to invoke a method declared for the class o Any module that mentions an instance variable declared in the class An interface is usually included with the #import directive: #import "ClassName.h" Example: #import "Person.h" This directive is identical to #include, except that it makes sure that the same file is never included more than once, eliminating the need for the header guards used in C and C++ An implementation file will normally import its counterpart interface. An interface file will normally import the interface for its superclass. If the superclass is defined in one of the Objective-C frameworks, the import syntax is slightly different: #import <FrameworkName/FrameworkName.h> Examples: #import <Foundation/Foundation.h> #import <UIKit/UIKit.h> Skeleton code provided by Xcode often includes some of the required import statements. Xcode projects may also include a precompiled header file that automatically imports one or more frameworks.

13 Referring to Other Classes An interface file may simply refer to class names without needing to import their interface files. Examples include: o An instance variable declaration o A method return value o A method parameter Declarations like this simply use the class name as a type and don t depend on any details of the class interface (its methods and instance variables). In this case, it is sufficient to declare the class names with Student, Employee; When the interface to a class is actually used (instances created, messages sent), the class interface must be imported. Typically, an interface file to declare classes it refers to, and the corresponding implementation file imports their interfaces.

14 Class Implementation A class implementation begins with the compiler and ends with the #import ClassName // method Methods are defined within a pair of braces. Before the braces, they re declared in the same manner as in the interface file, but without the semicolon. Example: #import Person - (int)age return age; - (void)setage:(int)value age =

15 Class Variables Objective-C does not have syntax for declaring a class variable, a data member that is shared among all instances of a class. You can fake this to some degree by declaring a global variable in your class implementation file and assigning it static linkage: static int enemycount = MyClass // Method The variable enemycount can be accessed by any method of MyClass (including class methods), but is not available to methods or functions declared in other implementation files.

16 Messaging Syntax When you want to call a method, you do so by messaging an object. A message is the method signature, along with the parameter information the method needs. All messages you send to an object are dispatched dynamically, thus facilitating the polymorphic behavior of Objective-C classes. Messages are enclosed by brackets ([ and ]). Inside the brackets, the object receiving the message is on the left side and the message (along with any parameters required by the message) is on the right. Some common message formats: o A message expression with no arguments: [receiver methodname] o A message expression with one argument: [receiver methodname:argument] o A message expression with two arguments: [receiver methodname:arg1 andarg:arg2] o A message expression with a parameter list argument: [receiver methodname:arg1, arg2, arg3, nil]

17 Message Expression Examples Student *student = [[Student alloc] init]; [student enrollat:@"niu" withmajor:@"computer Science"]; [student addclasses:@"csci 628", nil]; double gpa = [student gpa]; [student setgpa:3.85]; if ([student cangraduate]) // Do graduation-related stuff NSString *name = [[student advisor] name];

18 Terminology Message expression [receiver methodname:argument] [receiver methodname:arg1 andarg:arg2] Message [receiver methodname:argument] [receiver methodname:arg1 andarg:arg2] Selector [receiver methodname] [receiver methodname:argument] [receiver methodname:arg1 andarg:arg2] Method The code selected by a message.

19 Dot Syntax Objective-C 2.0 introduced dot syntax as a convenient shorthand for invoking accessor methods. Accessor methods get and set the state of an object, and typically take the form - (type)propertyname and - (void)setpropertyname:(type). float height = [person height]; float height = person.height; [person setheight:newheight]; person.height = newheight; This syntax can be used to replace nested messages: [[person child] setheight:newheight]; // exactly the same as person.child.height = newheight; Declared Properties A declared property provides a shorthand syntax for declaring a class s accessor methods and, optionally, implementing (attributes) data-type variablename;

20 Declared Properties You begin a property declaration with the You can then optionally provide a parenthesized set of property attributes that define the storage semantics and other behaviors of the property. Each property declaration ends with a type specification and a name. For (copy) NSString *title; This syntax is equivalent to declaring the following accessor methods: - (NSString *)title; - (void)settitle:(nsstring *)newtitle; The compiler will automatically synthesize the code for these methods, so you don t need to write them yourself. Read/Write Access readwrite (default) readonly Synchronization atomic (default) nonatomic ARC Storage Semantics strong (default) weak Declared Property Attributes Generate both setter and getter methods. Generate only the getter method. Include synchronization code to make setter and getter atomic operations. Do not include synchronization code for setter and getter, speeding up access. Take ownership of object. Do not take ownership of object; set to nil automatically when there are no remaining strong references to the object. Used with delegates, data sources, outlets, etc. to prevent strong reference cycles. assign No memory management; used with primitive data types. copy Create a copy of the object (which is then maintained with a strong reference). Object must conform to the NSCopying protocol. Often used for strings, arrays, dictionaries, etc. unsafe_unretained Used for an object of a class that does not support weak references.

21 Calling Your Own Methods An object can use the self pointer to invoke methods on itself: - (BOOL)canLegallyVote return (self.age >= 18); - (void)castballot if ([self canlegallyvote]) // Do voting stuff else NSLog(@"I am not allowed to vote!"); Similar to this in Java or C++.

22 Superclass methods Can also invoke superclass methods using super: - (void)dosomething // Call superclass implementation first [super dosomething]; // Then do our custom behavior int foo = bar; //...

23 BOOL Special Type Used to declare variables that will contain either a true or false value. Can be set to the built-in values YES (true, equivalent to 1) and NO (false, equivalent to 0). Example: BOOL isprime = NO; if (isprime)... while (isprime == YES)...

24 Object Types Object references are always declared as pointers: NSString *s; // Only a declaration; no instance is pointed to Declaring a reference pointer does not bring an instance into existence. You can also declare a dynamically-typed object using the data type id, a generic object type. Can be used to store an object of any type (both instances and class objects). id anobject; Just id, not id * (unless you actually want a pointer to a pointer to an object )

25 Instantiating Objects Many classes have factory methods, class methods that will allocate and initialize a new instance of the class for you and return a pointer to it: NSString *s = [NSString string]; // Create empty NSString object // Create NSString object from NSString literal NSString *s2 = [NSString stringwithstring:@"hello, world"]; Many classes have methods that simply return a pointer to an existing object: NSArray @"Three"]; // Array of 3 NSString objects NSString *s = [array objectatindex:2]; // s now points to the Allocating and initializing an instance from scratch is a two step process: allocation, then initialization. The steps must happen one right after the other, so this is usually done using two nested messages: NSString *name = [[NSString alloc] init]; Heap storage for the new object is allocated by the NSObject class method + (id)alloc. The object is initialized with an init method. Classes can have multiple, different initializers (with arguments) in addition to plain init. There s no method overloading, so the methods will have different names. If a class can t be fully initialized by plain init, it is supposed to raise an exception in init. NSObject s only initializer is init. If an initialization method has arguments, its name should still start with the word init. Some NSString examples: - (id)initwithcstring:(const char *)str encoding:(nsstringencoding)encoding; - (id)initwithdata:(nsdata *)data encoding:(nsstringencoding)encoding; - (id)initwithstring:(nsstring *)astring;

26 Writing Your Own Initializer Classes must have a designated initializer. This is the initializer that subclasses must use to initialize themselves in their designated initializer. A class may also have other convenience initializers. These will typically call the designated initializer as part of their code. Example: // Designated initializer - (id)initwithvalue:(int)avalue self = [super init]; // call the superclass designated initializer if (self) // initialize our subclass data members here value = avalue; return self; // Convenience initializer - (id)init self = [self initwithvalue:0]; // call our own designated initializer return self;

27 The nil Object Pointer You can assign an object reference pointer the special value nil (address 0, similar to NULL in C++ or null in Java) NSString *s = nil; You can test whether or not an object reference contains nil explicitly: if (nil == s) //... Or implicitly: if (!s) //... It is valid to send a message to a nil object reference unlike most languages, doing this will not cause a runtime error. Instead, the method you are trying to execute will generally do nothing or return nil: NSString *s = nil; // s is nil NSString *s2 = [s uppercasestring]; // now s2 is nil

28 Selectors A selector has data type SEL: SEL action = [button action]; [button setaction:@selector(start:)]; Conceptually similar to a pointer to a function. As shown previously, selectors include all parts of the method name and all colons; for example: - (void)setname:(nsstring *)name age:(int)age; would have the selector: SEL sel Working with Selectors You can determine if an object responds to a given selector: id obj; SEL sel if ([obj respondstoselector:sel]) [obj performselector:sel withobject:self] This sort of introspection and dynamic messaging underlies many Cocoa design patterns. - (void)settarget:(id)target; - (void)setaction:(sel)action;

29 Class Introspection You can ask an object about its class: Class myclass = [myobject class]; NSLog(@"My class is %@", [myobject classname]); Testing for general class membership (subclasses included): if ([myobject iskindofclass:[uicontrol class]]) // something Testing for specific class membership (subclasses excluded): if ([myobject ismemberofclass:[nsstring class]]) // something string specific These methods are largely defined in the NSObject class or in the NSObject protocol that it conforms to.

30 Object Identity vs. Object Equality Identity testing equality of the pointer values (addresses) if (object1 == object2) same object instance"); Equality testing object attributes if ([object1 isequal:object2]) equivalent, but may be different object instances"); Object Assignment Assigning one object reference to another does not make a copy of the object it makes a copy of the object s address. So now you have two object references pointing to the same object.

31 - description Method NSObject class implements the - description method: - (NSString *)description; Objects are represented in format strings using %@ (the format specifier for NSString). When an object appears in a format string, it is asked for its description: [NSString stringwithformat:@"the answer is %@", myobject]; You can log an object s description with NSLog: NSLog([anObject description]); NSLog(@"Object at index %d is %@", index, [anarray[index] description]); Your custom subclasses can override - description to return more specific information.

32 Protocols You can specify that a class conforms to a protocol in its interface HWAppDelegate : NSObject <UIApplicationDelegate> Multiple protocol names may be specified, separated by commas. You must then implement the required protocol methods in your class implementation. You can check whether an object conforms to a protocol by using the conformstoprotocol: method: for (id currentobject in myarray) if ([currentobject conformstoprotocol:@protocol(nscopying)]) // Call a required method from the NSCopying protocol directive used here takes a protocol name and produces a Protocol object, which is what the conformstoprotocol: method expects as its argument. You can get the compiler to check for for conformance with your variables by including the protocol name after the type name: id <NSCopying> currentobject; This tells the compiler that currentobject will reference objects that conform to the NSCopying protocol. If you assign a statically-typed object to currentobject that does not conform to the protocol, the compiler will issue a warning message. As when declaring a class implementation, you can list multiple protocol names separated by commas when specifying that an object should conform to a protocol.

33 Foundation Framework Classes NSObject Base class for practically every object in the ios SDK Implements memory management Implements introspection methods NSString International (any language) strings using Unicode. Used throughout ios instead of C language s char * type. NSString An NSString instance can not be modified! They are immutable. Usual usage pattern is to send a message to an NSString and it will return you a new one. [display settext:[[display text] stringbyappendingstring:digit]]; // same but with dot notation display.text = [display.text stringbyappendingstring:digit]; // class method display.text = [NSString stringwithformat:@"%g", number]; Tons of utility functions available (case conversion, URLs, substrings, type conversions, etc.).

34 NSMutableString Mutable version of NSString. Can do some of the things NSString can do without creating a new one (i.e., in-place changes). NSMutableString *mutstring = [[NSMutableString alloc] initwithstring:@"0."]; [mutstring appendstring:digit]; NSNumber Object wrapper around primitive numeric types like int, float, double, BOOL, etc. NSNumber *num = [NSNumber numberwithfloat:36.5]; float f = [num floatvalue]; @805.23e-10 Useful when you want to put these primitive types in a collection (e.g. NSArray or NSDictionary). NSValue Generic object wrapper for other non-object data types. CGPoint point = CGPointMake(25.0, 15.0); NSValue *val = [NSValue valuewithcgpoint:point];

35 NSData Bag of bits. Used to save/restore/transmit data throughout the ios SDK. NSDate Used to find out the time right now or to store past or future times/dates. See also NSCalendar, NSDateFormatter, NSDateComponents. NSArray Ordered collection of objects. Immutable. You cannot add objects to it or remove objects from it once it s created. NSArray value2, value2, ] Important methods: + (id)arraywithobjects:(id)firstobject,...; - (int)count; - (id)objectatindex:(int)index; - (void)makeobjectsperformselector:(sel)aselector; - (NSArray *)sortedarrayusingselector:(sel)aselector; - (id)lastobject; // returns nil if there are no objects in the array // (convenient) Bracket notation to access elements: mystring = [myarray objectatindex:2]; mystring = myarray[2];

36 NSMutableArray Mutable version of NSArray. - (void)addobject:(id)anobject; - (void)insertobject:(id)anobject atindex:(int)index; - (void)removeobjectatindex:(int)index; NSDictionary Hash table. Look up objects using a key to get a value. Immutable. NSDictionary : value1, key2 : value2, Keys are objects which must implement - (NSUInteger)hash and - (BOOL)isEqual:(NSObject *)obj Keys are usually NSString objects. Important methods: - (int)count; - (id)objectforkey:(id)key; - (NSArray *)allkeys; - (NSArray *)allvalues; Bracket notation to access values: myvalue = [mydictionary objectforkey:@"name"]; myvalue = mydictionary[@"name"];

37 NSMutableDictionary Mutable version of NSDictionary. NSSet - (void)setobject:(id)anobject forkey:(id)key; - (void)removeobjectforkey:(id)key; - (void)addentriesfromdictionary:(nsdictionary *)otherdictionary; Unordered collection of objects. Immutable. Important methods: - (int)count; - (BOOL)containsObject:(id)anObject; - (id)anyobject; - (void)makeobjectsperformselector:(sel)aselector; - (id)member:(id)anobject; // uses isequal: and returns a matching object // (if any) NSMutableSet Mutable version of NSSet. - (void)addobject:(id)anobject; - (void)removeobject:(id)anobject; - (void)unionset:(nsset *)otherset; - (void)minusset:(nsset *)otherset; - (void)intersectset:(nsset *)otherset;

38 Fast Enumeration Looping through members of a collection in an efficient manner Language support using for-in (similar to Java) Example: NSArray of NSString objects NSArray *myarray =...; for (NSString *string in myarray) // crash here if string is not an NSString double value = [string doublevalue]; Example: NSSet of id (could just as easily be an NSArray of id) NSSet *myset =...; for (id obj in myset) // do something with obj, but make sure you don t send it // a message it does not respond to if ([obj iskindofclass:[nsstring class]]) // send NSString messages to obj with impunity

39 Fast Enumeration Looping through the keys or values of a dictionary Example: NSDictionary *mydictionary =...; for (id key in mydictionary) // do something with key here id value = [mydictionary objectforkey:key]; // do something with value here

40 References APPLE INC., Object-oriented programming with Objective-C. APPLE INC., Programming with Objective-C. tion.html KOCHAN, S. G., Programming in Objective-C, Third Edition. Addison Wesley, Upper Saddle River, NJ.